1. Field of the Invention
The present invention relates to a hydraulically operated power transmission apparatus having a hydraulically operated transmission mechanism and a differential mechanism.
2. Description of the Relevant Art
Some known power transmission apparatus for use on riding-type working machines such as tractors include a hydraulically operated transmission mechanism having a hydraulic pump and a hydraulic motor which are interconnected by hydraulic passages and operable to continuously vary the transmission ratio or speed reduction ratio.
Power transmission apparatus comprising the hydraulically operated transmission mechanism of the type described above and a mechanical differential are disclosed in Japanese Laid-Open Utility Model Publication Nos. 61(1986)-139821 and 61(1986)-139822 and Japanese Laid-Open Patent Publication No. 63(1988)-258220, for example.
In the power transmission apparatus disclosed in Japanese Laid-Open Utility Model Publication Nos. 61(1986)-139821 and 61(1986)-139822, the hydraulically operated transmission mechanism and the differential mechanism are disposed independently of each other. Since the transmission and differential mechanisms have respective casings, the number of parts used is large, and the power transmission apparatus is large in size.
The power transmission apparatus disclosed in Japanese Laid-Open Patent Publication No. 63(1988)-258220 is also large in size and cannot easily be disassembled and assembled because the pump drive shaft of the hydraulic pump is positioned remotely from the hydraulic motor and extends perpendicularly to the output shaft of the hydraulic motor and the output shaft extending from the differential mechanism.
When the motor vehicle incorporating the above conventional hydraulically operated transmission apparatus runs downhill, the output shaft of the hydraulic motor is forcibly driven by the road wheels of the motor vehicle, applying forces to the swash plate of the hydraulic pump in a direction away from the neutral position. Then, a link mechanism coupled to the swash plate is released from a tension, and the swash plate tends to wobble about its pivot shafts for thereby allowing the speed of the motor vehicle to become unstable.